Steam generators of pressurized water nuclear reactors generally comprise a bundle consisting of a very large number of narrow-bore tubes bent into a U and fastened at each of their ends by crimping into a thick tube plate. The part of the steam generator which is situated below the tube plate forms a water box, in a first part of which the pressurized water coming from the reactor vessel is distributed into the bundle tubes, and in a second part of which, the water which has circulated in the tubes is recovered, to be re-directed, via a primary circuit conduit of the reactor, into the vessel enclosing the core, which consists of fuel assemblies responsible for the heating of the pressurized water. The steam generator feed water comes into contact with the outer surface of the bundle tubes, in the part of the steam generator which is situated above the tube plate.
The walls of the bundle tubes thus constitute a barrier between the pressurized water forming the primary fluid of the reactor and the feed water forming the secondary fluid. The primary water, which comes into contact the the fuel assemblies and the internal structures of the reactor vessel, contains radioactive products in more or less substantial quantities. The steam produced in the steam generator, by heating and vaporization of the feed water, is conveyed to a turbine associated with the nuclear reactor. It is therefore necessary to avoid any contact between the primary fluid and the secondary fluid, whose contamination could have highly detrimental consequences.
It is thus necessary to avoid as completely as possible the appearance of leakages through the walls of the bundle tubes of the steam generator and to repair as quickly as possible and effectively the bundle tubes whose wall has been perforated.
Because of the stresses of thermal and mechanical origin or the corrosion to which the bundle tubes are subjected, cracks can form in the walls of the bundle tubes, both on the primary side and on the secondary side, while the steam generator is in use.
In order to increase the lifespan of the steam generators and to ensure that this lifespan is similar or equal to that of the other components of the reactor, interventions or repairs must be carried out to the bundle tubes, during the periods of maintenance of the nuclear reactor.
To date, one of the most widely used techniques has consisted in taking the defective tube or tubes out of service by inserting and fastening a plug in a leakproof manner in the end of the tube which is flush with the tube plate and which the primary water enters. A disadvantage of this technique is that it reduces the exchange surface of the steam generator, the degree of such reduction increasing with the number of tubes taken out of service.
Processes for repairing the bundle tubes of a steam generator by lining these tubes have therefore been proposed, where the defect leads to a seal loss. A tubular sleeve or liner, with an external diameter which is slightly smaller than the internal diameter of the tube to be repaired and with sufficient length to overlap the defect, is inserted into the tube, through the entry face of the tube plate with which the ends of the tubes are flush. The liner itself is placed in the tube, inside the tube plate, so as to be flush with the entry face of the tube plate. The liner is then fastened to the interior of the tube by diametral expansion, followed by one or more operations permitting a leakproof connection to be made between the sleeve and the tube on each side of the defective region of the tube.
In particular, it is known to perform a diametral expansion of the liner in two regions in the vicinity of its ends, situated, in the case of one, within the thickness of the tube plate in the vicinity of its entry face and, in the case of the other, beyond the exit face of the tube plate, so that these two regions are situated on either side of the defective region of the tube to be repaired. The leakproof joint between the liner and the tube is produced by a roller-expanding operation on the liner in each of the regions which have been subjected to the diametral expansion.
All these operations require an intervention inside the water box of the steam generator, i.e., in a part of this generator which comes into contact with the primary water carrying radioactive products, while the reactor is in operation.
When these operations are performed manually, an operator has to enter the water box, where he is exposed to high radioactivity. The time for which the operator is present in the water box must therefore be reduced to the minimum, with the result that it is possible to perform only a small proportion of the tasks required for lining a tube, when this is done by an operator. This operator then becomes unavailable for continuing the operations of lining the tubes inside the water box, since his radiation dose reaches the upper limit which is generally permitted. In order to perform the operations on a steam generator, especially in the case where several tubes need to be lined a large number of competent and trained operators must therefore be available, and this considerably increases the manpower costs during the maintenance operations on the nuclear reactor.
Automatic appliances have therefore been proposed for performing at least partially the operations which are necessary for lining the tubes of a steam generator. Such appliances, which must perform the placing of the liner in a specified tube of the steam generator, then its expansion, its roller-expanding and/or its welding in a completely automatic manner, are extremely complicated and extremely bulky, since they need to comprise all the means necessary for the various successive stages of the lining operation and, in particular, all the means for driving the various tools used in succession. Furthermore, the capacity of such an appliance, insofar as the number of tubes which can be lined one after another without withdrawing the appliance from the water box is concerned, is, after all, limited by the maximum number of liners with which the appliance can be equipped at the time when it is inserted into the water box. Devices which have a barrel magazine containing several liners which can be presented in succession have been designed, to enable the latter to be inserted into the tubes to be lined. However, the capacity of such barrel magazines must be limited if excessive bulk and complexity of the appliance are to be avoided.